1. Introduction

Erbium laser at 1.5-1.6 µm via the ⁴I_13/2→⁴I_15/2 transition has many practical applications [1-4]. Yb³⁺ ions with large absorption cross-section around 980 nm, i.e. the emission wavelength of InGaAs diode laser, are generally co-doped as sensitizer to improve the pump efficiency. Er³⁺ and Yb³⁺ co-doped borate crystals, such as YAl₃(BO₃)₄ (YAB), GdAl₃(BO₃)₄ (GAB), YCa₄O(BO₃)₃ (YCOB), and GdCa₄O(BO₃)₃ (GCOB) crystals [1-4], have been demonstrated to be efficient gain media for the 1.5-1.6 µm laser. However, the YAB and GAB crystals can only be obtained by the flux method with a long growing period. Although the YCOB and GCOB can be grown by the Czochralski method, which is one of the techniques for growing large single crystals with good optical quality in a short period [5], the peak absorption cross-sections around 976 nm of the Er³⁺ and Yb³⁺ co-doped YCOB (0.9×10^-20 cm²) and GCOB (1.15×10^-20 cm²) crystals are small and the full widths at half the maximum (FWHMs) of the absorption bands (4 and 3 nm for YCOB and GCOB crystals, respectively) are narrow [3, 4, 6, 7]. The narrow absorption bands make precise temperature control of the pumping diode necessary and the small absorption cross-sections imply that a thicker medium is required for a certain dopant concentration, which will increase the internal loss and reduce the laser performance. The M₃Re₂(BO₃)₄ (M=Ca, Sr, Ba; and Re=Y, Gd, La) crystals belong to the orthorhombic system with space group Pc2₁n [8]. As the YCOB and GCOB, they melt congruently and can be grown by the Czochralski method [8]. It has been shown that Yb³⁺ ions doped in these crystals have larger absorption cross-section and broader absorption band around 976 nm than those of the YCOB and GCOB [5, 9, 10]. Therefore, the above disadvantage of small absorption cross-section and narrow absorption band can be overcome when the Er³⁺ and Yb³⁺ co-doped M₃Re₂(BO₃)₄ crystal is used as the gain medium.

The 1.56 µm laser and its second-harmonic generation (SHG) 780 nm laser, which is corresponding to the D₂ absorption line of ⁸⁷Rb, are important for some applications, such as laser cooling and trapping of atoms, atomic clock, metrology, and quantum communications and computing [11, 12]. However, up to date, single 1.56 µm laser oscillation has not been reported in the co-doped crystals and glasses because of the lack of suitable gain medium with good laser performance or gain peak at 1.56 µm. In this letter, high efficient quasi-cw 1.56 µm laser operation of borate crystal Sr₃Y₂(BO₃)₄ (SYB) is reported.

2. Spectral properties

An Er:Yb:SYB crystal with good optical quality was grown by the Czochralski method. The Er³⁺ and Yb³⁺ concentrations in the grown crystal were determined to be 0.9 at.% (6.9×10¹⁹ cm^-3) and 18.6 at.% (14.7×10²⁰ cm^-3), respectively, by inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon). The room temperature (RT) polarized absorption spectra recorded with a spectrophotometer (Lambda 900, Perkin-Elmer) in a range of 850–1070 nm are shown in Figure 1. In the figure X, Y, and Z represent the three principal axes of the optical indicatrix in order of increasing refractive index in these three directions n_X<n_Y<n_Z. The absorption spectra display weak polarized dependence although the crystal is biaxial. Therefore, the pump light can be utilized fully when the Er:Yb:SYB crystal is used as the gain medium because of the un-polarized characteristics for most of the commercial diode laser. The peak absorption wavelength and FWHM are 976 nm and 10 nm, respectively, for all the three polarizations. The FWHM is larger than those of Er:Yb:YCOB and Er:Yb:GCOB crystals [3, 4]. Therefore, the Er:Yb:SYB crystal is more suitable to be pumped by diode laser than the Er:Yb:YCOB and Er:Yb:GCOB after the emission bandwidth and wavelength temperature dependence of diode laser are considered. The peak absorption cross-sections are 1.63×10^-20, 1.87×10^-20, and 1.34×10^-20 cm² for E//X, E//Y, and E//Z, respectively, which are larger than those of Er:Yb:YCOB and Er:Yb:GCOB crystals [6, 7]. The larger absorption cross-section for the Er:Yb:SYB crystal implies that a thinner medium can be used at the same dopant concentration. Therefore, less internal loss and better coupling between large divergent beam of pumping diode laser and 1.5-1.6 µm laser beam in the thinner medium may be beneficial to better laser operation.

 

Fig.1. RT polarized absorption spectra of Er:Yb:SYB crystal in a range of 850–1070 nm

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Figure 2(a) shows the RT polarized absorption spectra recorded with the spectrophotometer (Lambda 900, Perkin-Elmer) in a range of 1400–1670 nm, which are related to the ⁴I_15/2→⁴I_13/2 transition of Er³⁺ ions in the crystal. The RT polarized fluorescence spectra for the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions under excitation at 976 nm were recorded using another spectrophotometer (FL920, Edinburgh). The resolution of both the absorption and emission spectra is 1.0 nm. The polarized emission cross-section spectra, which were calculated from the recorded fluorescence spectra by the Fuchtbauer-Ladenburg (FL) method [13], are shown in Figure 2(b). The peak emission cross-sections are 0.86×10^-2, 0.99×10^-20, and 0.86×10^-20 cm2 for E//X, E//Y, and E//Z, respectively, and all located at 1534 nm. The cross-sections are between the 2.0×10^-20 cm² of Er:Yb:YCOB crystal (E//Z) [14] and the 0.71×10^-20 cm² of Er:Yb:Li₆Y(BO₃)₃ crystal [15].

 

Fig. 2. RT polarized absorption (a) and emission spectra (b) of Er:Yb:SYB crystal in a range of 1400–1670 nm

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From the absorption and emission cross-sections, denoted as σ^abs and σ^em respectively, the gain cross-section curve can be calculated by σ^g(λ)=βσ^em(λ)-(1-β)σ^abs(λ), where β is the ratio of the number of Er³⁺ ions in the upper laser level ⁴I_13/2 to the total number of Er³⁺ ions. Due to the similarity of the gain curves for all the three polarizations, only the curves for E//Y with different β and the curves for E//X, E//Y, and E//Z with β=0.5 are shown in Figure 3 for clearness and comparison. For E//Y, the gain peak is located at 1.56 µm when β is larger than 0.3, which shows the possibility for realizing 1.56 µm laser oscillation. The gain curve for E//Y with β=0.5 is flat with FWHM of 70 nm. This value is larger than those of Er:Yb:YCOB crystal (58 nm) [16] and Er:Yb:YAB crystal (25 nm) [2] with the same value of β, in which picosecond pulse laser has been realized [17]. Therefore, tunable and short pulse laser may be realized in Er:Yb:SYB crystal.

 

Fig. 3. Gain curves of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions in Er:Yb:SYB crystal for E//Y with different β and for E//X, E//Y and E//Z when β is 0.5

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3. Laser experiments

Considering the anisotropy of the absorption and emission, a Z-cut 1.1-mm-thick plate of the grown Er:Yb:SYB crystal was used in the laser experiment. The uncoated plate was held in an aluminum mount and no special care was taken to ensure good thermal contact or cooling of the plate. For reducing the influence of the pump-induced thermal load on the laser performance and avoiding possible fracture of the plate, a 970 nm diode laser coupled by a fiber with 800 µm diameter core (FAP-980, Coherent) with pulse mode was used as the pump source. The pump pulse duration was 2 ms and the duty cycle was 2%. The pumping laser with power up to 18 W was focused on the plate with waist diameter of about 290 µm. The absorption coefficients of the Er:Yb:SYB crystal at 970 nm for E//X and E//Y are 8.9 and 9.9 cm-1, respectively. Considering that the pumping light is seriously divergent after it is focused in the gain medium, only little pumping light reflected by the out coupler re-passes the gain area and overlaps with the fundamental laser. Therefore, when only the single pass pumping is considered, about 60% of the incident pump power is absorbed by the plate. In the laser experiment an end-pumped hemispherical laser cavity was adopted. The flat input mirror has 90% transmission at 970 nm and 99.8% reflectivity at 1.5-1.6 µm. Two output couplers with the same radius curvature of 50 mm and different transmissions of 1.0% and 1.5% at 1.5-1.6 µm were used. The reflectivities of the two output couplers at 970 nm were higher than 98%. The cavity length was kept at about 50 mm.

Figure 4 shows the measured fundamental laser output power as a function of the absorbed pump power for the two output couplers. Because the duty cycle of the quasi-cw pulse laser was 2%, the values in the figure are the measured ones multiplied by 50. For the output coupler with transmission of 1.0%, the absorbed pump threshold power was 3.9 W, the slope efficiency was 16% and about 1.13 W output power was obtained when the absorbed power was 10.86 W. When the output coupler transmission increased to 1.5%, the absorbed pump threshold power and slope efficiency increased to 4.26 W and 20%, respectively, and output power up to 1.3 W was achieved when the absorbed power was 10.86 W. It can be found from Figure 4 that the output power was not saturated when the absorbed power was 10.86 W, so higher output power may be achieved when the pump power increases. Furthermore, only two output couplers with different transmissions of 1.0% and 1.5% were used in the experiments; therefore, more efficient laser output may be realized if the transmission of the output coupler is optimized.

 

Fig. 4. Laser output power at 1.5-1.6 µm as a function of absorbed pump power at 970 nm

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The laser emission spectra for the two output couplers, recorded with a monochromator (Triax 550, Jobin-Yvon) when the absorbed power was 10.86 W, are shown in Figure 5. It is worth noting that efficient 1.56 µm laser is achieved and the oscillating laser is always around 1.56 µm at any pump powers when the transmission of output coupler is 1.5%. Furthermore, the laser wavelength has a slight red shift of about 5 nm when the transmission of output coupler decreases to 1.0%. The shift of laser wavelength with the reduction of output coupler transmission can be explained by the E//Y gain curves with different values of β shown in Figure 3 [18]. All the laser beams are linearly polarized with E//Y, which is important for the application of nonlinear frequency conversion and can also be explained by the gain curves shown in Figure 3. The gain cross-section for E//Y is larger than that for E//X at a certain β and more favorable for the laser oscillation.

 

Fig. 5. Spectra of the Er:Yb:SYB laser at 1.5-1.6 µm when the absorbed pump power is 10.86 W and output coupler transmissions are: (a) T=1.0%, (b) T=1.5%.

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4. Conclusion

1.3 W quasi-cw laser output around 1.56 µm with a slop efficiency of 20% was achieved in Er:Yb:SYB crystal grown by the Czochralski method. By means of the intracavity frequency doubling technique, the linearly polarized 1.56 µm laser can be further effectively converted into 780 nm laser, which has some important applications as mentioned above. It is worth noting that the absorption cross-section of the Er:Yb:SYB crystal at 976 nm is twice as that at the pump wavelength of 970 nm in this work (see Figure 1). Therefore, if a pump source around 976 nm is used, the laser performance of a thinner Er:Yb:SYB medium could be improved. Furthermore, large single Er:Yb:SYB crystal can be growth easily in a short period via the Czochralski method. Combining with the suitability for diode pumping, flat and broad gain curve, and good laser performance, it can be concluded that this crystal is a potential gain medium for tunable and short pulse laser around 1.56 µm.